Hot water is widely needed in domestic buildings, commercial buildings, and industry for heating, bathing, industrial processes, etc. Gas-fired boilers and electric heaters are mostly used to supply hot water. A heat pump water heater (HPWH) is another type of equipment to supply hot water. The use of HPWH in domestic and light commercial applications is growing rapidly due to its low environmental impact as HPWH uses renewable energy as a heat source.
As shown in FIG. 1, a heat pump water heater unit (HPWH) mainly includes a system including a compressor 1, a condenser 2, an expansion device 3, and an evaporator 4. The evaporator can be, for example, an air-source type or a water source type. Working fluid (refrigenat) is charged into the HPWH system and then circulated through the system using a vacuum pump. A heat exchange occurs between water and the working fluid in the condenser.
During the circulation cycle of HPWH, the compressor 1 discharges high-pressure hot working fluid vapor into the condenser 2. The hot working fluid (side 2′) loses heat to water (side 2″) in the condenser 2 and condenses into liquid. The water absorbs the heat from the working fluid becoming hot water. The liquid working fluid then flows through the expansion device 3 and is throttled into a low-pressure two-phase working fluid. The low-pressure two-phase working fluid enters the evaporator 4 where the fluid absorbs heat and then evaporates into a low-pressure vapor which is then suctioned into the compressor 1. The working fluid flows through the HPWH system in direction 5. Water flows through the condenser 2 in water flow direction 6.
A new requirement was established for HPWHs due to problems with legionnella contamination. HPWHs now need to provide water temperature above 60° C. (ASHRAE Guideline 12-2000). In some cases, this temperature needs to be 65° C. due to a temperature drop in the connecting lines. An example of technical requirements for some HPWHs is as follows: Reliability: 110° C. Maximum compressor discharge temperature, 28 bar maximum discharge pressure. Performance: High efficiency to reduce environmental impact, acceptable capacity to fulfill user needs.
Current working fluids used in heat pump water heaters include HCFC-22, HFC-134a, HFC-410A, HFC-407C and HFC-417A. However, HCFC-22, HFC-410A and HFC-407C are unsuitable to supply 60-65° C. hot water and can only be used in existing compressor technologies due to excessive temperature and pressure at the compressor discharge. HCFC-22 is being phased out worldwide because it attacks the ozone layer and can not supply 60-65° C. hot water due to its very high discharge temperature and pressure. HFC-407C can not supply 60-65° C. hot water because its discharge temperature and pressure are very high. HFC-410A can not supply 60-65° C. hot water because its discharge temperature and pressure are very high.
HFC-134a, and HFC-417A can supply 60-65° C. hot water with existing compressor technologies, but their performances are marginal. HFC-134a can supply 60-65° C. hot water with low discharge temperature and pressure, but it has very small capacity and requires a large compressor. Moreover the capacity decreases greatly when the ambient temperature is less than 5° C. HFC-417A is a popular refrigerant for heat pump water heater units to supply 60-65° C. hot water since its discharge temperature and pressure are almost safe for existing compressors (discharge pressure exceeds 28 MPa) and its capacity and efficiency are marginal.